Servo-controlled drive for machine tools and the like



April 3, 1962 J. K. ROYLE 3,027,703

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 19585 Sheets-Sheet 1 A5 A 5 SW5 1 ROTARY-TO-L/NEAR V cogyveersz o p S52v0SIGNAL M Mr, STAT! NAEY F IGJ.

5mm :IZLC d M SIGNAL Q 4 RM ACTIVE \eauzv-ro-Lwme CONVEETEQ PASS/V5,MOV/NG FIG.4.

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INVENIOR I JOSEPH KENNETH ROYLE ATTORNEYS April 3, 1962 J. K. ROYLE3,0273%:

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 19585 Sheets-Sheet 2 v I Q V SEEVO SIM r- H FIG 3 A .H 1 I 1 M U pl 5' j m vf Aer/vs 552 v0 eomev-m-u/vsu CONVEQTEQ SIGNAL PASSIVE, smnamev F IG 2S50v0 M $/GNAL Vm) Aer/v5, EL 6 Q T? I P15 :1

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PASS'IVE, STAT/OM42) INVENTOR JOSEPH KENNETH ROYLE ATTORNEYS April 3,1962 J. K. ROYLE 3,027,703

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 19585 Sheets-Sheet 3 7777i TABLE DIGITAL r0 VALVE F (5. 4, i ANALOGUEmum/2,4102 5 I I nan/47012 CONVERTER 4 FIG.9.

l v INVEN'IIDR JOSEPH gggNNETH Rom r ATTORNEYS April 3, 1962 I J. K.ROYLE SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE 5Sheets-Sheet 4 Filed May 27, 1958 FIG. l5.

3-PHA5E 4.6.

PHASE I PHASE 2 PHASE INVENTOR JOSEPH KENNETH ROYLE FIG. l6.

Mfwonumrs April 3, 1962 J. K. ROYLE 3,027,703

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 1958q K 5 Sheets-Sheet 5 INVENTOR- JOSEPH KENNETH BYLE ATTORNEYS UnitedStates Patent fi 3,027,703 Patented Apr. 3, 1962 3,027,763SERVO-CONTROLLED DRIVE FUR MACHINE TOOLS AND THE LIKE Joseph KennethRoyle, Heat-on Moor, Stoclrport, England,

assignor to National Research Development Corporation, London, England,a British corporation Filed May 27, 1958, Ser. No. raaasa Claimspriority, application Great Britain May 29, 1957 8 Claims. (Cl. 60-6)This invention relates to apparatus for driving a first object along apredetermined path in relation to a second object and is particularlyapplicable to the propulsion of the slides of machine tools along theirslideways though the invention is not confined to such applications.

According to the invention there is provided apparatus for driving afirst object in relation to a second object along a predetermined path,comprising two driving mechanisms coupled together and to the firstobject and the second object in such a manner that movements of thefirst object relative to the second object along the said path aredependent upon the algebraic sum of the individual driving actions ofthe two driving mechanisms, the first driving mechanism being capable ofa relatively high driving acceleration over only a relatively shortrange of driving action and the second driving mechanism being capableof a relatively long range of driving action but being arranged toprovide only a relatively low driving acceleration, means, responsive toa signal representing the motion required of the first object inrelation to the second object along the said path, for actuating thefirst driving mechanism and means, operable before the first drivingmechanism, in moving from the middle of its range of driving action, hasreached an end of its range of driving action in responding to a signalas aforesaid, for actuating the second driving mechanism in such a senseas to supplement the driving action of the first driving mechanisminitiated by the said signal, whereby the first driving mechanism isprevented from reaching the end of its range of driving action.

According to the invention there is further provided apparatus fordriving a first object in relation to a second object along apredetermined path, having a first driving mechanism comprising ahydraulic piston and cylinder arrangement and a second driving mechanismcomprising a rotary driving motor coupled to a device for transformingrotary motion derived from the said motor, into motion in the directionof the said path, the two driving mechanisms being coupled together andto the first object and the second object so that relative movementbetween the two objects along the said path is dependent upon thealgebraic sum of the driving actions of the two driving mechanisms, thehydraulic piston and cylinder arrangement having a short stroke andbeing controlled by a high performance hydraulic valve, the seconddriving mechanism having a range of driving action which is long inrelation to the said stroke but being arranged to provide a drivingacceleration which is low in relation to that of the hydraulic pistonand cylinder arrangement, a closed servo loop for controlling therelative motion between the two objects in the direction of the saidpath comprising means for receiving signals characteristic of desiredrelative motion between the said objects along the said path, means forreceiving signals characteristic of relative motions taking placebetween the said objects along the said path, means for comparing thesaid signals so received and continuously generating an error signalrepresenting the difference between the said signals, an actuator forthe hydraulic valve, means for continuously operating the actuator inaccordance with the instantaneous value of the error signal,

cooperating members coupled respectively to the piston and cylinder ofthe said hydraulic arrangement engaging when the piston reaches apredetermined position between the middle and an end of its stroke,means for energising the motor of the second driving mechanism in such asense as to supplement the driving action of the hydraulic arrangementwhen the said members engage as aforesaid and de-energising the motorwhen the members have moved out of engagement.

The invention will be more readily understood from the followingdescription of certain embodiments thereof illustrated in theaccompanying drawings in which:

FIGS. 1 to 8 inclusive are diagrammatic representations of certainvariants of the invention, I

FIG. 9 is an elevation of a first embodiment of the invention,

FIG. 10 is a sectional plan of the said first embodiment,

FIG. 11 is a cross-sectional side elevation of the said firstembodiment,

FIG. 12 is a cross-section to an enlarged scale of a part of the saidfirst embodiment,

FIG. 13 is a detailed underside view to an enlarged scale of a part ofthe said first embodiment,

:FIG. 14 is a schematic diagram of a servo control system for use withthe invention,

FIG. 15 is a diagram of a motor control circuit for use with theinvention,

FIG. 16 is a diagram of a second motor control circuit for use with theinvention.

For convenience of description the first driving mechanism willhereinafter be referred to as the quick drive and the second drivingmechanism will be hereinafter referred to as the slow drive but theseterms should not be considered as in any way defining or limiting theproperties of the two said mechanisms.

The invention finds its principal uses in systems where the relativepositions, velocity or acceleration (or any combination of the three),for between two objects, along a predetermined path, is required to beunder control of a command signal of which a characteristic is variableaccordingto the relative position, velocity or acceleration (orcombination of the three) required of the two objects.

Systems of this type, commonly called servo systems, present thedifficulty that a driving mechanism having a quick response to a changeof the command signal will tend to have a small range of driving actionand conversely a driving mechanism having a large range of drivingaction will tend to have a slow response to a change of the commandsignal.

This may be illustrated by the following examples:

A hydraulic piston and cylinder under the control of a well designedvalve can respond extremely quickly to a sudden change in a commandsignal applied to an actuator controlling the valve but if the cylinderis long this advantage is lost due to the compliance or compression ofthe hydraulic fluid contained in the cylinder. The compressibility ofthis volume of fluid reduces the stiffness of the system so that theresonant frequency of the inertia load is lowered.

The stiffness of a long hydraulic cylinder can be increased byincreasing the diameter of the bore but this in turn increases thevolume of the fluid to be moved which requires a larger valve with aslower response.

The quick response of a hydraulic piston and cylinder arrangement istherefore only obtainable when the stroke is short.

To provide a relatively large movement between the two objects variousmechanisms, many of which involve rotating elements, are available. Anexample of the latter is the well known lead screw and nut, one elementbeing rotated by a rotary motor. Another example is a rack and worm.These mechanisms can be designed to provide great stiffness combinedwith a long stroke but it is correspondingly difiicult to procure quickresponse without complication and expense. For instance, if the rotatingelement is driven by an electric motor, the armature cannot be rapidlyaccelerated without the application of considerable power. Thisdifficulty becomes more acute the heavier the loads encountered.

A long hydraulic piston and cylinder combination is another instance ofa driving mechanism suitable for providing relatively large movementbetween the two objects but it has already been explained that such adrive lacks stilfness unless it has a large diameter.

The invention combines the virtues of both types of driving mechanismand avoids their limitations.

FIGURES l to 7 inclusive, show various kinematically equivalentcombinations of a quick drive and a slow drive for providing linearrelative motion between a first object and a second object, theseobjects being illustrated as a machine tool work table riding onslideways anchored to the fixed parts of the machine tool, and the saidfixed parts respectively. The quick drive is represented as a shortstroke hydraulic ram Q, controlled by a valve V operated by an actuatorA responsive to command signals and the slow drive is represented as abar working in combination with a rotary-to-linear converter RLC (suchas a lead screw and nut respectively or a rack and worm respectively)driven by a motor M.

In FIG. 1 the quick drive is carried by the work table and has amoveable member attached to the bar, the latter cooperating with arotary-to-linear converter RLC anchored to the slideways and rotated bya motor similarly anchored.

In FIG. 2 the quick drive is again carried by the work table and itsmoveable member is attached to the bar. The bar rotates so that themoveable member of the quick drive must also rotate unless a rotaryjoint is interposed between them. If the moveable member is the pistonof a hydraulic piston and cylinder arrangement, the piston can be givensufiicient clearance to enable it to rotate in the cylinder withoutundue friction. The bar cooperates with a non-rotating or passiverotary-tolinear converter RLC which is anchored to the slideways and thebar is rotated by a motor M carried by the work table, via a splinedjoint which permits endwise movement of the bar to the extent of thestroke of the quick drive.

In FIG. 3 the quick drive is floating, with its moveable member coupledto the work table. A non-rotating or passive rotary-to-linear converterRLC is anchored to the quick drive and cooperates with a bar rotated bya motor M anchored to the slideways. In practice the quick drive wouldride on slideways parallel to the slideways for the work table.

In FIG. 4 the quick drive Q is anchored to the slideways and itsmoveable member is coupled to a non-rotating or passive rotary-to-linearconverter RLC cooperating with a bar rotated by a motor M anchored tothe worktable. This arrangement has the advantage, where a hydraulicdevice is used for the quick drive, that it can be supplied by rigidpiping.

In FIG. 5 both the quick drive Q and the motor M of the slow drive areanchored to the slideways and the quick drive moveable member is coupledto the bar of the slow drive either directly or through a rotarycoupling and the bar co-operates with a non-rotating or passiverotary-to-linear converter RLC anchored to the work table. Where thereis no rotary coupling between the bar and the moveable member of thequick drive the latter must rotate. The motor M drives the bar throughgearing and a splined joint which permits endwise movement of the bar tothe extent of the stroke of the quick drive. With this arrangement allpower supplies are coupled without the need for flexible leads.

In FIG. 6, the quick drive Q is anchored to the slideways, the motor Mof the slow drive and a rotating or active rotary-to-linear converterRLC are connected to the moveable member of the quick drive and thisassembly would in practice be mounted on additional slideways parallelto the main slideways supporting the work table and providing freemovement along the additional slideways to the extent of the stroke ofthe quick drive. The rotary-to-linear converter cooperates with anonrotating or passive bar anchored to the work table.

In FIG. 7 the quick drive Q the motor M of the slow drive and a slidingcoupling for rotating the bar of the slow drive, are all anchored to thework table. The bar of the slow drive is coupled to the moveable memberof the quick drive so that the latter must rotate unless a rotarycoupling is introduced between them. The bar of the slow drivecooperates with a non-rotating or passive rotary-to-linear converter RLCanchored to the slideways.

in the form where the quick drive is capable of relative linearmovements between its driving members over a limited length of strokeand where the slow drive incorporates a rotary motor and arotary-to-linear converter.

FIG. 8 illustrates a rotary version of the invention in which a worktable journalled for rotation in bearings (not shown) is driven by aslow drive in the form of a worm wheel coupled to the work table (shownas machined from the under side of the rim of the work table). This wormwheel cooperates with a Worm driven by the motor M of the slow drivewhich are mounted on a subtable which is also journalled for rotationcoaxial with the work table in bearings (not shown). The quick drive Qis anchored to fixed parts of the machine and can rotate the subtablecarrying motor M to and fro over a limited angle by means of aconnecting rod. This arrangement is the rotary equivalent of FIG. 6.

A practical embodiment of the invention will now be described inrelation to FIGURES 9 to 15 of the accompanying drawings.

FIG. 9 is an elevation of a milling machine incorporating the invention.

The machine has a main casting 1 providing a horizontal platform 2supporting slideways 3 upon which a work table 4 may ride from right toleft and vice versa, as shown in the figure. Behind the slideways 3, avertical pillar 5 fixed to main casting 1 supports a beam 6 overhangingthe work table 4 and beam 6 has on its underside slideways not visiblein the drawing, upon which rides a vertical tool spindle mounting 7which is capable of horizontal movement on the slideways of the beam 6in a direction normal to the plane of the paper. The slideways 3 and theslideways on the beam 6 permit relative movement between a tool carriedon the spindle 8 and a work piece fixed to work table 4, along two axesat right angles to one another.

The work table 4 is moved along slideways 3 by means of a motor 9driving a worm 10 (shown in dotted lines in FIGURE 9) which cooperateswith a rack 11 (shown mainly in dotted lines in FIGURE 9). Motor 9 iscoupled to worm 10 via a worm and worm wheel gear train 12 and a shaft13 (shown in dotted lines in FIG- URE 9). Shaft 13 is coupled to wormIt) by gearing 14 not shown in FIGURE 9 but visible in FIGURE 10.

Rack 11 rides in guides 15 in the underside of work table 4 (visible inFIGURE 11) and is coupled to the work table via the piston rod 16 of ahydraulic piston and cylinder arrangement 17 the cylinder of which isbolted to Work table 4.

FIGURE 10 shows the milling machine in plan, sectioned in a horizontalplane running through the centre of worm 10. FIGURE 11 is a verticalsection of the milling machine along line 18, looking in the directionof the arrows.

Shaft 13 is supported at one end by bearings in a hous- FlGS l to 7illustrate principal varients of the invention ing 19, fixed rigidly toslideways 3, which accommodates the gear train 12.

At its other end shaft 13 is supported in bearings secured to the floorof the structure of slideways 3. Worm 10 is journalled for rotation in acradle 21 and this cradle is capable of rotating bodily about the axisof shaft 13 which passes through bearing bushes 22 in cradle 21 whichprovide a pivot for the latter. Special provisions are taken to preventendwise movement of shaft 13, worm 10 and cradle 21. These provisions,which are omitted to simplify the drawing, may take the form ofconventional thrust bearings or may include a thrust bearing loaded by ahydraulic thrust arrangement. Cradle 21 has a lug 23 on the side remotefrom shaft 13 which rests on a hydraulic jack unit 24. Jack unit 24 issupplied with hydraulic fluid at a suitable pressure and forces the ing23 upwards so that cradle 21 is urged to rotate about shaft 13 clockwisein relation to FIGURE 11 whereby Worm 10 is forced upwards intoengagement with rack 11 for the elimination of back-lash.

FIGURE 12 shows the hydraulic piston and cylinder combination 17 insection to an enlarged scale.

A cylinder body 25 is secured to the end of work table 4 by means ofbolts such as bolt 26. A piston 27 slides in the bore linear 28 and iscoupled to rack 11 by piston rod 16, which passes through a glandedcylinder end 29, by means of the screw-threaded extension 30. The otherend of the cylinder is closed by a cylinder end 31, both cylinder endsbeing secured to the cylinder body 25 by means of bolts such as 32. Theside of the cylinder to the right of piston 27, as seen in FIGURE 12,commun cates with a hydraulic valve 33 via a port 34 in cylinder end 31.

Valve 33 has a bore 35 in which rides a spool 36 having two controllinglands 37 and 38 and two sealing lands 39 and 40. The lands 37 and 38co-operate with annular ports 41 and 42 grooved out of the surface ofbore 35 and communicating via supply ports 43 and 44 with unions (notshown) for the attachment of pipes leading to the high pressure and lowpressure sides respectively of a hydraulic pressure supply system. Thecontrolling lands 37 and 38 are dimensioned so that, when the spool isin the central position they do not completely isolate ports 41 and 42from one another whereby there is a small flow of hydraulic fluidbetween supply ports 43 and 44. This arrangement is commonly referred toas underlap and makes for stable operation of the valve at smallopenings.

The side of the cylinder to the left of piston 27, as seen in FIGURE 12communicates via a port 45 in cylinder end 29, with a union (not shown)for attachment of a pipe leading to the high pressure side of thehydraulic pressure supply system. The full pressure of the said systemis therefore constantly applied to the left hand face of piston 27 butover an effective area which is reduced by the cross sectional area ofpiston rod 16. The pressure on the other side of piston 27 operates uponits full area however and is thus able to overcome the pressure on theleft of piston 27 and force the latter to the left when the valve 33 isoperated in the appropriate sense (Le. upwards as seen in FIGURE 12).Conversely, when the valve 33 is operated in the opposite sense theright hand side of piston 27 is acted upon by the low pressure of thehydraulic system, which is overcome by the high pressure to the left ofthe piston even though it works upon a smaller elfective area of pistonsurface.

A groove 46 surrounds the outer end of piston rod 16 and is incommunication via port 47 with a union (not shown) for attachment to thelow pressure side of the hydraulic supply system, whereby oil leakingalong the piston rod is scavenged.

Spool 36 is operated axially by an actuator which, for the sake ofsimplicity, is omitted from the drawing. This actuator is of a typeresponsive to command signals characteristic of movements required to bemade by work table 4 along slideways 3. Such actuators are well known inthe art, a suitable type being of the type commonly known as a torquemotor. This type of actuator works on the same principle as a centrezero moving coil electric meter movement but is of robust constructionand capable of exerting a substantial force via a finger correspondingto the pointer of a meter.

In operation, the command signals produce movements of piston 27 whichcause corresponding movements of work table 4, along slideways 3, inrelation to rack 11,

which, when motor 9 is at rest, is locked against axial movement by worm10, the slant angle of which is such as to make the rack and wormcombination virtually irreversible in the sense that loads applied tothe rack 11 cannot rotate the worm 10.

Rack 11 carries along one side, cams such as 48 in FIGURE 11 and shownin detail in FIGURE 13 which gives an enlarged underside view of part ofthe work table 4 and rack 11. Fast with the work table 4 are switchessuch as 49 each having a trigger member 50 cooperating with a cam 48.

At least two switch-and-cam sets 48/49 are provided at convenient sitesalong rack 11 and they are so placed that one switch engages its camwhen the piston 27 moves off-centre in one direction and the otherswitch engages its cam when piston 27 moves off-centre in the otherdirection. It is arranged that the operation of the switches 49 controlthe motor 9 in such a way that the action of the hydraulicpiston/cylinder combination 17 is supplemented by movement of rack 11before piston 27 reaches the end of its stroke.

For this to be effective it is in general necessary for the signalsapplied to the actuator for valve 33 to be subject to a feed-back servosystem. An example of such a system is schematically illustrated inFIGURE 14.

FIGURE 14 shows a record tape 51, for instance a magnetic record tape,carrying signals characteristic of the motions required to be executedby work table 4. Tape 51 is carried on spools 52 and passed byconventional magnetic record play-back methods over a read ing head 53.The reproduced command signals pass to a comparator 54 which has anotherinput from apparatus which originates signals characteristic of motionsmade good from time to time by Work table 4 along slideways 3. FIGURE 14illustrates one arrangement of this type in which an elongated opticaldiffraction grating 55 is attached to and moves with the work table anda smaller grating 56 is anchored to the slideways, the two gratingsbeing so placed and aligned that the small grating is narrowly separatedfrom and overlies some part of the large grating in all positions of thework table along its slideways. The directions of the rulings of the twogratings are relatively inclined at a small angle so that alternate darkand light bands (so-called moir fringes) are seen when looking throughthe two superimposed gratlngs at a light source. These bands areapproximately normal to the direction of the lines of the two gratingsand move in the direction of their breadth when the small grating movesalong the large grating on movement of the work table along itsslideways. When an optical slit, parallel to the longitudinal axes ofthe bands, is introduced into the light path, a photo electric cell suchas that indicated at 57 in FIGURE 14, trained on the light emerging fromthe gratings and the slit, has an output (ideally sinusoidal inwave-form) which fluctuates according to the movement of the bands.

The pitch of the rulings on the diffraction gratings 55 and 56 may be ofthe same order of magnitude as the limits of accuracy to which themachine is to be controlled. A movement of the work table equal to thedistance between two adjacent rulings produces amovement of the bands(or moir fringes) such that one band occupies the place previously heldby its immediate neighbour. As the bands and their spacing is many timesgreater than those of the grating lines the device provides a sensitivemeasure of work table movements, and the output of the light cell passesthrough one minimum and one maximum for each said movement of the worktable.

Various proposals have been made for distinguishing between diiferentdirections of movement of the work table, which results in differentdirections of movement of the bands or moir fringes, but it is notconsidered necessary to describe such methods. It is sufiicient to saythat light cell 57 gives an output which fluctuates in time with smallincrements ofmovement of the work table of predetermined length and thatthe direction of movement can be distinguished. If the command signalsfrom pickup 53 are in the form of signal elements each representing oneof the said small increments of movement of the work table and havingone form for one direction of movement and another form for the oppositedirection of movement, the two inputs into comparator 54 can be comparedand any difference can be arranged to represent the discrepancy at anyinstant between the movements required of the work table and themovement actually made good. This difierence or error signal, derivedfrom fluctuating indications, will be in incremental or digital form. Toapply such an error signal to valve 33 a digitalto-analogue converter 58is interposed between the comparator 54 and the valve actuator 59'. Theanalogue signal applied to actuator 59 may be a voltage corresponding tothe number of units of error held in comparator 54 at any instant.

Such a voltage applied to valve actuator 59 will operate valve 33 tocause relative movement between piston 27 and cylinder 17 and as thepiston is held by the rack, the cylinder will move and with it the worktable. If the command signals from pick-up 53 call for continuousmovement of work table 4 there will be a continuous error signal in 54since, as soon as a cancelling signal is received from light cell 57 tocancel one command signal, the error is registered again on receipt of anew command signal. When the piston moves a predetermined distance fromits central position in the cylinder one of the switches 49 is operatedand motor 9 starts up. The worm and rack 10, 11 then drive the worktable in the same direction as it is already being driven by piston 27and the two superimposed motions cause a momentary excess of signalsfrom light cell 57 which result in signals of reversed sense beingpassed from comparator 54. The result is that the valve 33 is operatedin the reverse sense to move piston 27 back towards its centralposition. If the table is moving at the correct speed under control ofmotor 9 and the worm and rack 10, 11, the piston will not move far inthis reverse direction and may not move far enough to disengage theswitch 49, in which case the motor will remain in operation until therequired work table movement is exceeded. Since however, the motor,worm, rack combination 9, 10, 11 must be capable of centering piston 27even in the presence of continued command signals in the same sense, thesystem must be arranged so that the maximum speed of the motor, worm,rack combination exceeds the maximum speed required of the work table.Therefore, if the motor 9 remains switched on, the work table will soonbe moving faster than it should which will result in reverse signalsfrom comparator 54 and compensatory movement of the piston 27 towardsits central position. The motor 9 will be switched oil before thisposition is reached by piston 27 but will continue to revolve by inertiafor some little time so that the piston may, in compensating for anycontinued excess speed of the work table, overshoot its central positionand operate the other switch 49 so as to reverse the motor. In practicethere will be some hunting as between the piston and the motor. Thiswill not matter in many cases since the sensitivity of the hydraulicpiston and cylinder assembly 17 to signals received via valve actuator59 will be sufficient to correct momentary discrepancies between theactual and the required movements of the work table, within acceptablelimits of accuracy.

An arrangement to minimise this hunting will be described later however.

FIGURE 15 shows a method of actuating motor 9 under control of the twoswitches 49. These switches are represented by their respective(normally open) contacts SB and SF one of which is connected in seriesbetween a battery supply and a relay CB and the other of which isconnected in series between the battery supply and a relay CF. Theserelays may be heavy duty contactors of the type used in conventionalmotor starter units. In FIG- URE 16, motor 9 is shown as a three phasealternating current motor. Such a motor may be reversed by transposingthe first and third phase connections from the supply mains to themotor. The three phase wires from the supply mains are connected to themotor, first via three normally open contacts cfll, cfZ, and cf3 ofrelay CF and secondly via normally open contacts cbl, CM and cb3 ofrelay Cii. As the two switches 49 cannot be simultaneously operated onlyone of the relays CB and CF at a time, can be operated. When CF isoperated the phase I, phase 2 and phase 3 phase wires from the supplymains are connected to the upper, middle and lower terminals of themotor respectively. With CB operated the phase 1, phase 2 and phase 3wires are connected to the lower, middle and upper terminals of themotor respectively.

The cams 48 are so shaped and located that when piston 27 movesoil-centre sufiiciently to bring one of the switch triggers 5% intoengagement with the cooperating cam 48, the trigger remains operated bythe cam during further movements of the piston in the same direction, tothe end of its stroke.

The points in the stroke of piston 27, when switches 49 operate, must beset in from the ends of the piston stroke by an amount which will bedictated by the acceleration capabilities of motor 9 and also by itsslowing down time. To minimise the risk of piston 27 reaching the end ofits stroke before motor, worm, rack combination 9, 10, 11 of the slowdrive can come to its aid, the switching points should be fairly closeto the central position. When the slow drive is moving the work tabletoo fast, or in the wrong direction (for instance when the commandsignals call for an abrupt reversal of direction on the part of the worktable), it is desirable that motor 9 should be switched off at an earlypoint in the recovery stroke of the piston 27 to avoid a prolongedover-run of the slow drive. This calls for a wide separation of theswitching points from the central position. A suitable compromise mustbe made between these two conflicting requirements.

To minimise hunting, additional cam and switch combinations such as 48,49 may be provided. These will be located so as to operate on movementsof the piston nearer to the ends of its stroke than the operating pointsof switches SF and SB. These additional switches have contacts each ofwhich can operate a single relay and this relay has three pairs ofnormally open contacts each pair being connected in parallel with aresistance, and one of these resistances being connected in series withone of the three phase wires from the supply mains on the left of FIG.16. Normally, when only switch SP or SE is operated, motor 9 will beenergised at reduced power. When one of the additional switches isoperated, however, the three series resistances will be short circuitedand full power will be applied to motor 9.

In all embodiments of the invention it is preferable that the completechain of linkages from one to the other of the two objects between whichrelative movement is required, should be free from back-lash andelasticity since any lost motion due to these factors constitutes anabsolute limit to the accuracy with which the driving mechanisms cancontrol the relative movements between the two objects. It isnevertheless permissible to have a certain amount of back-lash andelasticity present in such linkages (indeed it is scarcely possible toeliminate them entirely in practice), so long as the total lost motionremains substantially below the limits of accuracy with 9 which therelative motion between the two objects is to be controlled.

The slow drive should preferably be irreversible in the sense that itcannot yield under loads imposed upon it by the driving action of thequick drive.

A certain amount of reversibility is permissible however so long as suchreversibility operates at an extremely low mechanical advantage and solong as it involves the acceleration of some element of substantialmass. This may be illustrated in connection with the embodiment of theinvention shown in FIGURES 9, 10, ll, and 12. Whereas the worm It can berotated by motor 9 to propel rack 11, an endwise thrust on the rack 11cannot cause the worm 10 to revolve to any significant extent becausethe angles of the mating teeth are such as to provide an overwhelmingmechanical disadvantage to such reversed movement. If the mating teethwere cut at a more oblique angle, however, the worm 10' might be capableof turning in the presence of high endwise thrusts imparted to rack 11by the quick drive 17. This would not seriously impair the operation ofthe machine, however, so long as the worm or elements rigidly coupled toit had high inertia so as to retard any reversed motion of the rack 11.Suppose, for instance, that the quick drive is acting under commandsignals calling for movement of the work table 4 to the right as seen inFIGURE 9. If worm 10 were slowly to start turning in response to the endthrust on rack 11, then the rightwards movement of work table 4 wouldslowly fall behind the rate of movement called for by the commandsignals. This would produce (in a monitoring feed-back system such asthat illustrated in FIGURE 14) a slowly rising deficit of signals fromthe monitoring equipment (i.e. from light cell 57) as against theincoming command signals (i.e. from reading head 53). In response tothis deficit, quick drive 17 would increase the speed of its drivingaction to the right and would soon operate one of the switches 49 toenergise motor 9. Even if motor 9 took some time to reach full speed, itWould develop enough torque to arrest the rotation of worm 10 earlierthan this and in any event the acceleration of worm 10 due to relativelyhigh inertia would be slow enough to give motor 9 time to start andreverse the movement of piston 27 before it reached the end of itsstroke. Without this inertia worm 10 might yield so quickly that thequick drive could exhaust its stroke before motor 9 had picked upsufficiently to come to the assistance of the quick drive.

The invention has been illustrated by embodiments in which the quickdrive is in the form of a hydraulic piston and cylinder arrangement andthe slow drive is in the form of a device such as a lead screw and nutor a rack and worm, driven by a rotary motor. The quick drive and theslow drive can of course take other forms within the scope of theinvention. For instance, in an extremely high performance equipment thequick drive may take the form of an electro-mechanical transducer suchas a ma-gneto-strictive transducer and a suitable slow drive for usewith this quick drive might take the form of a short stroke hydraulicpiston and cylinder arrangement such as is used for the quick drive inthe embodiment of the invention illustrated in FIGS. 9 to 11. Thisarrangement would have an extremely rapid response and a correspondinglyshort range of movement, and if this range of movement proved to beinadequate it could form part of a triple drive with a still slowerdrive such as the slow drive illustrated in FIGS. 9 to 11 operating intandem with the magneto-strictive transducer and the hydraulic pistonand cylinder; each of the relatively slower drives being energised so asto prevent the next quicker drive from exhausting its stroke.

I claim:

1. Apparatus for driving a first object in relation to a second objectalong a predetermined path, comprising two driving mechanisms coupledtogether and to the first object and the second object in such a mannerthat movements of the first object relative to the second object alongthe said path are dependent upon the algebraic sum of the individualdriving actions of the two driving mechanisms, the first drivingmechanism being capable of a relatively high driving acceleration overonly a relatively short range of driving action and the second drivingmechanism being capable of a relatively long range of driving action butbeing arranged to provide only a relatively low driving acceleration,means, responsive to a signal representing the motion required of thefirst object in relation to the second object along the said path, foractuating the first driving mechanism and means, operable before thefirst driving mechanism, in moving from the middle of its range ofdriving action, has reached an end of its range of driving action inresponding to a signal as aforesaid, for actuating the second drivingmechanism in such a sense as to supplement the driving action of thefirst driving mechanism initiated by the said signal, whereby the firstdriving mechanism is prevented from reaching the end of its range ofdriving action.

2. Apparatus as claimed in claim 1 in which the means for actuating thesecond driving mechanism is operated when two parts of the first drivingmechanism, which execute relative movement to provide the driving actionof the first driving mechanism, have moved to the relative positionsthey occupy when the first driving mechanism is at a predetermineddistance from an end of its range of driving action.

3. Apparatus as claimed in claim 2 in which the means for actuating thesecond driving mechanism comprise two mutually cooperating members onecoupled to one of the said parts of the first driving mechanism and theother to the other of the said parts, the second driving mechanism beingactuated under control of the said two members.

4. Apparatus as claimed in claim 3 in which the second driving mechanismcomprises an electric motor and the said two members cooperate toconnect the said motor to a source of electric current.

5. Apparatus as claimed in claim 3 in which the second driving mechanismcomprises a rotary motor coupled to a device for transforming rotarymotion derived from the motor into motion in the direction of the saidpath.

6. Apparatus as claimed in claim 3 in which the said two members comeinto engagement to actuate the second driving mechanism when the saidparts of the first driving mechanism have moved to the relativepositions they occupy when the first driving means is at a predetermineddistance from an end of its range of driving action and in which thesaid two members disengage to terminate the actuation of the seconddriving mechanism when the said parts of the first driving mechanism,having moved to the relative positions they occupy when the firstdriving means is at the said predetermined distance from an end of itsdriving range, leave those relative positions on the return of the firstdriving means towards the middle of its range of driving action.

7. Apparatus for driving a first object in relation to a second objectalong a predetermined path, having a first driving mechanism comprisinga hydraulic piston and cylinder arrangement and a second drivingmechanism comprising a rotary driving motor coupled to a device fortransforming rotary motion derived from the said motor, into motion inthe direction of the said path, the two driving mechanisms being coupledtogether and to the first object and the second object so that relativemovement between the two objects along the said path is dependent uponthe algebraic sum of the driving actions of the two driving mechanisms,the hydraulic piston and cylinder arrangement having a short stroke andbeing controlled by a high performance hydraulic valve, the seconddriving mechanism having a range of driving action which is long inrelation to the said stroke but being arranged to provide a drivingacceleration which is low in relation to that of the hyddraulic pistonand cylinder arrangement, a closed 1 1. servo loop for controlling therelative motion between the two objects in the direction of the saidpath comprising means for receiving signals characteristic of desiredrelative motion between the said objects along the said path, means forreceiving signals characteristic of relative motions taking placebetween the said objects along the said path, means for comparing thesaid signals so received and continuously generating an error signalrepresenting the diiference between the said signals, an actuator forthe hydraulic valve, means for continuously operating the actuator inaccordance with the instantaneous value of the error signal, cooperatingmembers coupled respectively to the piston and cylinder of the saidhydraulic arrangement engaging when the piston reaches a predeterminedposition between the middle and an end of its stroke, means forenergising the motor of the second driving mechanism in such a sense asto supplement the driving action of the hydraulic arrangement when thesaid members engage as aforesaid and de-energising the motor when themembers have moved out of engagement.

8. Apparatus as claimed in claim 7 in which the said transforming devicecomprises a rack and Worm combination.

References Cited in the file of this patent UNITED STATES PATENTS1,961,090 Smith May 29, 1939 2,367,492 Pickett et a1 I an. 16, 19452,426,910 Wilson 2 Sept. 2, 1947 2,535,909 Ernst Dec. 26, 1950 2,684,443Tiball i July 20, 1954 2,785,353 Fenmore v Mar. 12, 1957 2,835,142Foster a- May 20, 1958 2,867,759 Comstock Jan. 6, 1959 2,907,937 ApgarOct. 6, 1959 FOREIGN PATENTS 760,321 Great Britain Oct. 31, 1956 OTHERREFERENCES Publication: Aircraft Production, July 19-55, pages 267- 272,Computer-Controlled Machine-Tools, by-D. T. N. Williamson.

